Rotary seals generally comprise two relatively rotatable members. In a labyrinth-type rotary seal, one such member typically includes one or more circumferential teeth which are coaxially positioned with respect to a circumferential sealing surface on the second member. Seals of this type are used to restrict the flow of a gas stream between cavities formed by the stationary and rotating members without impeding the rotational movement of the rotating member.
A disadvantage of seals of this type occurs when the thermal and mechanical forces acting on the seal change. Typically, one member responds quickly to the change in temperature resulting in thermally induced radial growth. At the same time, the other member may heat more slowly thereby thermally growing at a slower rate. Similarly, the rotating member will undergo mechanical growth when operating due to centrifugal forces acting thereon. Such transient conditions may affect the steady state running clearance.
At equilibrium operating temperature, the separation or gap between rotating and stationary members is designed to be fixed at a typically small value. In practice, large variations occur in the steady state running clearance due in part to transient conditions as well as the difficulty in calculating thermal/mechanical stresses at steady state. These large variations make accurate prediction of the leakage flow rate through the seal very difficult.